Phase shifting optical device with dopant barrier

ABSTRACT

A doped barrier region included in an optical phase shifter is disclosed. In one embodiment, an apparatus according to embodiments of the present invention includes a first region of an optical waveguide and a second region of the optical waveguide. The second region of the optical waveguide includes a higher doped region of material and a lower doped region of material. An insulating region disposed between the first and second regions of the optical waveguide is also included. A first portion the higher doped region is disposed proximate to the insulating region. A dopant barrier region is also included and is disposed between the higher and lower doped regions of the second region of the optical waveguide.

REFERENCE TO PRIOR APPLICATIONS

This application is a divisional of, and claims priority under 35 U.S.C.§ 120 from, U.S. patent application Ser. No. 10/872,982, filed Jun. 21,2004, now issued as U.S. Pat. No. 7,035,487 B2, entitled “Phase ShiftingOptical Device With Dopant Barrier”.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to optics and, morespecifically, the present invention relates to phase shifting opticalbeams.

2. Background Information

The need for fast and efficient optical-based technologies is increasingas Internet data traffic growth rate is overtaking voice traffic pushingthe need for optical communications. Transmission of multiple opticalchannels over the same fiber in the dense wavelength-divisionmultiplexing (DWDM) systems and Gigabit (GB) Ethernet systems provide asimple way to use the unprecedented capacity (signal bandwidth) offeredby fiber optics. Commonly used optical components in the system includewavelength division multiplexed (WDM) transmitters and receivers,optical filter such as diffraction gratings, thin-film filters, fiberBragg gratings, arrayed-waveguide gratings, optical add/dropmultiplexers, lasers and optical switches. Optical switches may be usedto modulate optical beams. Two commonly found types of optical switchesare mechanical switching devices and electro-optic switching devices.

Mechanical switching devices generally involve physical components thatare placed in the optical paths between optical fibers. These componentsare moved to cause switching action. Micro-electronic mechanical systems(MEMS) have recently been used for miniature mechanical switches. MEMSare popular because they are silicon based and are processed usingsomewhat conventional silicon processing technologies. However, sinceMEMS technology generally relies upon the actual mechanical movement ofphysical parts or components, MEMS are generally limited to slower speedoptical applications, such as for example applications having responsetimes on the order of milliseconds. In electro-optic switching devices,voltages are applied to selected parts of a device to create electricfields within the device. The electric fields change the opticalproperties of selected materials within the device and the electro-opticeffect results in switching action. Electro-optic devices typicallyutilize electro-optical materials that combine optical transparency withvoltage-variable optical behavior.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention is illustrated by way of example and notlimitation in the accompanying figures.

FIG. 1 is a cross-section illustration of one embodiment of an opticaldevice including a dopant barrier disposed between higher and lowerdoped regions in accordance with the teachings of the present invention.

FIG. 2 is a plot illustrating one embodiment of a relationship ofoptical absorption versus physical dopant concentration for bulklarge-grain Boron-doped polysilicon in accordance with the teachings ofthe present invention.

FIG. 3 is a block diagram illustration of one embodiment of a systemincluding an optical transmitter and an optical receiver with an opticaldevice including one embodiment of an optical phase shifter according toembodiments of the present invention.

FIG. 4 is a block diagram illustration of one embodiment of an opticalmodulator including a Mach Zehnder Interferometer (MZI) having oneembodiment of an optical phase shifter according to embodiments of thepresent invention.

DETAILED DESCRIPTION

Methods and apparatuses for phase shifting an optical beam with anoptical device with reduced optical loss are disclosed. In the followingdescription numerous specific details are set forth in order to providea thorough understanding of the present invention. It will be apparent,however, to one having ordinary skill in the art that the specificdetail need not be employed to practice the present invention. In otherinstances, well-known materials or methods have not been described indetail in order to avoid obscuring the present invention.

Reference throughout this specification to “one embodiment” or “anembodiment” means that a particular feature, structure or characteristicdescribed in connection with the embodiment is included in at least oneembodiment of the present invention. Thus, appearances of the phrases“in one embodiment” or “in an embodiment” in various places throughoutthis specification are not necessarily all referring to the sameembodiment. Furthermore, the particular features, structures orcharacteristics may be combined in any suitable manner in one or moreembodiments. In addition, it is appreciated that the figures providedherewith are for explanation purposes to persons ordinarily skilled inthe art and that the drawings are not necessarily drawn to scale.

FIG. 1 is a cross-section illustrating generally one embodiment of anoptical device 101 including a dopant barrier 115 in accordance with theteachings of the present invention. In one embodiment, optical device101 is a semiconductor-based optical device that is provided in a fullyintegrated solution on a single integrated circuit chip. Embodiments ofthe disclosed optical devices can be used in a variety of high bandwidthapplications including multi-processor, telecommunications, networkingas well as other high speed optical applications such as optical delaylines, switches, modulators, add/drops, or the like.

As shown in FIG. 1, optical device 101 includes a first region ofmaterial 103 and a second region of material. In the embodiment depictedin FIG. 1, the second region of material is illustrated as materialregions 105 and 111 including a dopant barrier 115 disposed betweenmaterial regions 105 and 111 in accordance with the teachings of thepresent invention. In one embodiment, dopant barrier 115 may include athin layer of silicon nitride, silicon dioxide or other suitablematerial, which in one embodiment may be deposited or grown in betweenthe material regions 105 and 111 of optical waveguide 127 in accordancewith the teachings of the present invention.

In one embodiment, one or more of material 103 and material regions 105and 111 include semiconductor material, such as for example silicon. Forexplanation purposes, material 103 and material regions 105 and 111 willbe described in this disclosure as including semiconductor materials.Other suitable materials may be utilized in accordance with theteachings of the present invention. In another embodiment, materialregion 111 may include a material having an index of refraction similarto the index of refraction of material region 105, such as silicon, andprovide good transmission of infrared light. For example, in anembodiment in which material region 105 is silicon, material region 111may include large-grain undoped polysilicon, a dielectric such ashafnium oxide (HfO₂), or other suitable materials.

In one embodiment, dopant barrier 115 helps to concentrate the dopantconcentration of the second region of semiconductor material insemiconductor material region 105 such that semiconductor materialregion 105 has a higher dopant concentration than semiconductor materialregion 111. In one embodiment, semiconductor material region 111 issubstantially undoped polysilicon or has a substantially low dopingconcentration. In one embodiment, the semiconductor material regions mayinclude silicon, polysilicon, or other suitable types of semiconductormaterial.

In the illustrated embodiment, semiconductor material region 103 isillustrated as having a plurality of portions 103A, 103B and 103C andsemiconductor material region 105 is illustrated as having a pluralityof portions 105A, 105B and 105C. In one embodiment, the portions 103Band 103C of semiconductor material region 103 have a higher dopantconcentration than portion 103A of semiconductor material region 103.Similarly, in one embodiment, the portions 105B and 105C ofsemiconductor material region 105 have a higher dopant concentrationthan portion 105A of semiconductor material region 105. In oneembodiment, contacts 117 and 119 are coupled to semiconductor materialregion 105 at portions 105B and 105C, respectively. Similarly, in oneembodiment, contacts 121 and 123 are coupled to semiconductor materialregion 103 at portions 103B and 103C, respectively.

In one embodiment, contacts 117 and 119 are coupled to receive a signalV_(SIGNAL) and contacts 121 and 123 are coupled to ground. In anotherembodiment, contacts 117 and 119 are coupled to ground and contacts 121and 123 are coupled to receive a signal V_(SIGNAL). In one embodiment,semiconductor material 103 also includes n-type dopants andsemiconductor material 105 includes p-type dopants. In anotherembodiment, semiconductor material 103 also includes p-type dopants andsemiconductor material 105 includes n-type dopants. The polarities ofthe dopants and voltages are provided in this disclosure for explanationpurposes and that the polarities of the dopants and correspondingvoltages may be modified or reversed in accordance with the teachings ofthe present invention.

In one embodiment, an insulating region 113 is disposed between andproximate to semiconductor material regions 103 and 105. In oneembodiment, insulating region 113 includes for example SiON, SiO₂, oranother suitable type of insulating material. As illustrated in FIG. 1,one embodiment of optical device 101 is fabricated on asilicon-on-insulator (SOI) wafer and therefore includes a buriedinsulating layer 107 and a layer of semiconductor material 109. In oneembodiment, buried insulating layer includes for example SiO₂ or anothersuitable type of insulating material between semiconductor material 103and 109 layers. Insulating region 113 disposed between semiconductormaterial regions 103 and 105, such that a complementary metal oxidesemiconductor (CMOS) capacitive type structure is formed. As shown inFIG. 1, charge carriers in charge regions 133 are formed proximate toinsulating region 113 in semiconductor material regions 103 and 105,which form the plates of a capacitor while the insulating region 113provides the insulator between the “plates.” In one embodiment, theconcentration of charge carriers in charge regions 133 is modulated inresponse to V_(SIGNAL) in accordance with the teachings of the presentinvention.

In one embodiment, an optical waveguide 127 is included in opticaldevice 101, through which an optical beam 125 is directed along anoptical path. In the embodiment illustrated in FIG. 1, waveguide 127 isa rib waveguide including a rib region 135 and a slab region 137. In oneembodiment, optical beam 125 includes infrared or near infrared light.For example, in one embodiment, optical beam 125 has a wavelength nearapproximately 1.3 μm or 1.55 μm. In the embodiment illustrated in FIG.1, the optical path along which optical beam 125 is directed is along anaxis that is parallel to the axis of the optical waveguide of opticaldevice 101. In the example shown in FIG. 1, the optical path andtherefore optical beam 125 are shown to propagate along a directiongoing through, or coming in and out of, the page.

As summarized above, one embodiment of semiconductor material region 103is grounded through contacts 121 and 123 and semiconductor materialregion 105 is coupled to receive V_(SIGNAL) through contacts 117 and119. In one embodiment, contacts 113, 115, 117 and 119 are metalcontacts that are coupled to semiconductor material regions 103 and 105at locations outside the optical path or optical mode of optical beam125. Similarly, the higher doped portions 103B, 103C, 105B and 105C ofsemiconductor regions 103 and 105 are also disposed at locations outsidethe optical path or optical mode of optical beam 125. The application ofV_(SIGNAL) to optical waveguide 127, as shown in FIG. 1, results in themodulation of free charge carriers in charge regions 133, which isproximate to insulating region 113 and through which optical beam 125 isdirected.

In one embodiment, portion 103A of semiconductor material 103 ismoderately doped n-type silicon having a doping concentration of, forexample, approximately 3×10¹⁶ cm⁻³. In one embodiment, portion 105A ofsemiconductor material 105 is moderately doped p-type polysilicon havinga doping concentration of, for example, approximately 1×10¹⁷ cm⁻³. Insuch an embodiment, higher doped portions 103B and 103C are heavilydoped with n++ type dopants and higher doped regions 105B and 105C areheavily doped with p++ type dopants. For example, higher doped regions105B and 105C in one embodiment are heavily-doped p-type polysiliconhave a doping concentration of approximately 1×10¹⁹ cm⁻³. In oneembodiment, the higher doped regions may be made of semiconductormaterials such as silicon, polysilicon, silicon germanium, or any othersuitable type of semiconductor material. In one embodiment, theinclusion of higher doped portions 103B, 103C, 105B and 105C helpimprove the electrical coupling of metal contacts 117, 119, 121 and 123to semiconductor material regions 103 and 105 in accordance with theteachings of the present invention. This improved electrical couplingreduces the contact resistance between metal contacts 117, 119, 121 and123 and semiconductor material regions 103 and 105, which improves theelectrical performance of optical device 101 in accordance with theteachings of the present invention.

In one embodiment, a buffer of insulating material 129 and a buffer ofinsulating material 131 are also included in an optical device 101 inaccordance with the teachings of the present invention. As shown in FIG.1, buffer 129 is disposed between contact 117 and the optical path oroptical mode of optical beam 125. Buffer 131 is disposed between contact119 and the optical path or optical mode of optical beam 125. In oneembodiment, buffers 129 and 131 are made of materials having lowerrefractive indexes than the refractive index of the core of waveguide127. As a result, buffers 129 and 131 serve as cladding so as to helpconfine optical beam 125 to remain within waveguide 127. In theembodiment illustrated in FIG. 1, buried insulating layer 107 alsoserves as cladding so as to help confine optical beam 125 to remainwithin waveguide 127. In one embodiment, buffers 129 and 131 also serveas optical and electrical isolators so as to optically isolate metalcontacts 117 and 119 from optical beam 125 as well as electricallyisolate material 103 from material 105 and electrically isolate thecontacts coupled to waveguide 127 from the optical electric field guidedfrom optical beam 125.

In operation, optical beam 125 is directed through optical waveguide 127along an optical path through charge regions 133. In one embodiment,V_(SIGNAL) is applied to optical waveguide 127 at material region 105 tomodulate the free charge carrier concentration in charge regions 133proximate to insulating region 113. The applied voltage from V_(SIGNAL)changes the free charge carrier density in charge regions 133, whichresults in a change in the refractive index of the semiconductormaterial in optical waveguide 127.

In one embodiment, the free charge carriers in charge regions 133 mayinclude for example electrons, holes or a combination thereof. In oneembodiment, the free charge carriers may attenuate optical beam 125 whenpassing through. In particular, the free charge carriers in chargeregions 133 may attenuate optical beam 125 by converting some of theenergy of optical beam 125 into free charge carrier energy. Accordingly,the absence or presence of free charge carriers in charge regions 133 inresponse to in response to V_(SIGNAL) will modulate optical beam 125 inaccordance with the teachings of the present invention.

In one embodiment, the phase of optical beam 125 that passes throughcharge regions 133 is modulated in response to V_(SIGNAL). In oneembodiment, the phase of optical beam 125 passing through free chargecarriers in charge regions 133, or the absence of free charge carriers,in optical waveguide 127 is modulated due to the plasma optical effect.The plasma optical effect arises due to an interaction between theoptical electric field vector and free charge carriers that may bepresent along the optical path of the optical beam 125 in opticalwaveguide 127. The electric field of the optical beam 125 polarizes thefree charge carriers and this effectively perturbs the local dielectricconstant of the medium. This in turn leads to a perturbation of thepropagation velocity of the optical wave and hence the index ofrefraction for the light, since the index of refraction is simply theratio of the speed of the light in vacuum to that in the medium.Therefore, the index of refraction in optical waveguide 127 of opticaldevice 101 is modulated in response to the modulation of free chargecarriers in charge regions 133. The modulated index of refraction in thewaveguide of optical device 101 correspondingly modulates the phase ofoptical beam 125 propagating through optical waveguide 127 of opticaldevice 101. In addition, the free charge carriers in charge regions 133are accelerated by the field and lead to absorption of the optical fieldas optical energy is used up. Generally the refractive indexperturbation is a complex number with the real part being that partwhich causes the velocity change and the imaginary part being related tothe free charge carrier absorption. The amount of phase shift φ is givenbyφ=(2π/λ)ΔnL  (Equation 1)with the optical wavelength λ, the refractive index change Δn and theinteraction length L. In the case of the plasma optical effect insilicon, the refractive index change Δn due to the electron (ΔN_(e)) andhole (ΔN_(h)) concentration change is given by:

$\begin{matrix}{{\Delta\; n} = {{- \frac{e^{2}\lambda^{2}}{8\pi^{2}c^{2}ɛ_{0}n_{0}}}( {\frac{{b_{e}( {\Delta\; N_{e}} )}^{1.05}}{m_{e}^{*}} + \frac{{b_{h}( {\Delta\; N_{h}} )}^{0.8}}{m_{h}^{*}}} )}} & ( {{Equation}\mspace{14mu} 2} )\end{matrix}$where n₀ is the nominal index of refraction for silicon, e is theelectronic charge, c is the speed of light, ε₀ is the permittivity offree space, m_(e)* and m_(h)* are the electron and hole effectivemasses, respectively, b_(e) and b_(h) are fitting parameters.

In one embodiment, the dimensions of optical waveguide 127 are designedto accommodate a single mode for optical beam 125. For instance, in oneembodiment, the width W_(R) of the rib region 135 of optical waveguide127 is approximately 1.8 μm, the height H_(R) of the rib region 135 ofoptical waveguide 127 is approximately 1.0 μm and the height H_(S) ofthe slab region 137 of optical waveguide 127 is approximately 0.9 μm. Inone embodiment, the height H_(B) of the buried insulating layer 107 isapproximately 0.9 μm the thickness T_(G) of the insulating region 113 isapproximately 6 nm. In one embodiment, the thickness T_(B) of dopantbarrier 115 is less than or equal to approximately 10 nm and the widthW_(U) of semiconductor material region 111 is approximately 1.3 μm. Inone embodiment, the resulting stack of dopant barrier 115, semiconductormaterial 105 and insulating region 113 has a stack thickness T_(S) ofapproximately 0.10 to 0.25 μm. These dimensions of one embodiment areprovided in this disclosure for explanation purposes and that otherdimensions may be utilized in accordance with the teachings of thepresent invention.

Optical insertion loss is often dominated by the absorption andscattering that occurs in the doped semiconductor materials. Byconcentrating the dopants in semiconductor material 105 with dopantbarrier 115, optical insertion loss of optical waveguide 127 is reducedin accordance with the teachings of the present invention. As a result,device performance of optical device 101 as given for example by theratio of device speed/device optical loss is improved.

For instance, FIG. 2 is a diagram showing optical absorption or lossversus physical dopant concentration for bulk large-grain Boron-dopedpolysilicon. When moving from low doping to high doping, theconductivity increases more rapidly than the optical loss. As can beobserved from the plots showing boron electrically active concentration(SPR) and boron physical dopant concentration (SIMS), there is an orderof magnitude improvement in activation by concentrating dopant inaccordance with the teachings of the present invention. Therefore, afigure of merit for the material, for example electrical conductivity ofmaterial/optical loss of material, increases with increasing dopantconcentration. Since dopant barrier 115 has the effect of concentratingdopants in semiconductor material 105 proximate to insulating region 113according to embodiments of the present invention, an improved ratio ofelectrical conductivity of material/optical loss of material is realizedin accordance with the teachings of the present invention.

In addition, with dopant barrier 115 disposed between semiconductormaterial regions 105 and 111 as shown, a substantial portion of thedoped semiconductor material of semiconductor material 105 is disposedalong a periphery of optical waveguide 127 in lower-intensity regions ofthe optical mode of optical beam 125 in optical waveguide 127. Indeed,as can be observed from the embodiment shown in FIG. 1, the portions ofsemiconductor material 105 that are proximate to buffers 129 and 131that are disposed along the periphery of optical waveguide 127 aresubstantially outside the optical path or optical mode of the opticalbeam 125 in accordance with the teachings of the present invention. As aresult, the effectiveness of dopant barrier 115 disposed betweensemiconductor material regions 105 and 111 as shown is further enhancedbecause optical loss in optical waveguide 127 is further reduced becausethe optical mode intensity decreases substantially near the periphery ofoptical waveguide 127 near the interface between semiconductor material105 and buffers 129 and 131.

FIG. 3 illustrates generally a block diagram of one embodiment of asystem including an optical transmitter and an optical receiver with anoptical device including an optical phase shifter according toembodiments of the present invention. For example, optical device 305may include optical device 101 of FIG. 1 or optical modulator 401 ofFIG. 4. In particular, FIG. 3 shows optical system 301 including a laser303 and an optical receiver 307. In one embodiment, optical system 301also includes an optical device 305 optically coupled between laser 303and optical receiver 307 through optical fiber 339 and optical 341. Inone embodiment, laser 303 transmits a laser beam 325 that is received byoptical device 305 through an optical conduit 339. In one embodiment,optical receiver is optically coupled to optical device 305 to receivelaser beam 325 through an optical conduit 341. In one embodiment,optical conduits 339 and 341 may include for example optical fibers,optical waveguides, free space or other suitable optical conduits.

In one embodiment, system 301 may be included in a single computersystem with laser 303, an optical device 305 and optical receiver 307being included in internal components of the computer system. Forexample, in one embodiment, system 301 may be a computer system, such asfor example a personal or laptop computer, with optical device 305included in a processor 343 of the computer system and optical receiver307 being included in for example an internal card 345 of the computersystem, such as for example a video controller card, a network interfacecard, memory or the like. In such an embodiment, optical communicationsare provided between the processor 343 that includes optical device 305and the internal card 345 that includes optical receiver 307. In anotherembodiment, system 301 may be included in a single chip or chipset withlaser 303 and optical receiver 307 being internal components of the chipor chipset. In still another embodiment, system 301 may be included in acommunications network with laser 303 and optical receiver 307 beingincluded in separate components of the communications network.

In one embodiment, optical device 305 may include for example a devicesuch as optical device 101 described above to phase shift laser beam 325in response to signal V_(SIGNAL). In such an embodiment, optical device305 may serve as for example an optical delay. In another embodiment,optical device 305 may be employed in an optical amplitude modulator orthe like. In various embodiments according to the teachings of thepresent invention, optical device 305 can be designed with scaled downwaveguide dimensions to operate at high speeds without excessive opticalloss as discussed above.

FIG. 4 illustrates generally one embodiment of an optical modulator 401that can be included in optical device 305 of FIG. 3. As shown in thedepicted embodiment, optical modulator 401 includes an optical phaseshifter 403 in at least one of the two arms optically coupled betweencascaded Y-branch couplers of a Mach-Zehnder Interferometer (MZI)configuration 405 disposed in semiconductor material. In one embodiment,optical phase shifter 403 is similar to an embodiment of optical device101 described above.

In operation, an optical beam 425 is directed into an input of MZIconfiguration 405. Optical beam 425 is split such that a first portionof the optical beam 425 is directed through one of the arms of the MZIconfiguration 405 and a second portion of optical beam 425 is directedthrough the other one of the arms of the MZI configuration 405. As shownin the depicted embodiment, one of the arms of the MZI configuration 405includes optical phase shifter 403, which adjusts a relative phasedifference between the first and second portions of optical beam 425 inresponse to signal V_(SIGNAL). In one embodiment, the first and secondportions of optical beam 425 are then merged in the semiconductorsubstrate such that optical beam 425 is modulated at the output of MZIconfiguration 405 as a result of constructive or destructiveinterference. In one embodiment, as shown, one of the arms of the MZIconfiguration 405 includes an optical phase shifter 403. In anotherembodiment, both of the arms of the MZI configuration 405 may include anoptical phase shifter 403 in accordance with the teachings of thepresent invention. In various embodiments according to the teachings ofthe present invention, optical phase shifter 403 can be designed withscaled down waveguide dimensions and non-uniform doping concentrationsand profiles operate at high speeds such as for example 10 GHz andbeyond without excessive optical loss is discussed above.

In the foregoing detailed description, the method and apparatus of thepresent invention have been described with reference to specificexemplary embodiments thereof. It will, however, be evident that variousmodifications and changes may be made thereto without departing from thebroader spirit and scope of the present invention. The presentspecification and figures are accordingly to be regarded as illustrativerather than restrictive.

1. A method, comprising: directing an optical beam along an optical paththrough an optical waveguide; modulating a concentration of free chargecarriers in a charge modulated region along the optical path to phaseshift the optical beam; and concentrating dopants in the opticalwaveguide proximate to an insulating region disposed between first andsecond regions of the optical waveguide and along a periphery of theoptical waveguide to confine the dopants in the optical waveguide with adopant barrier region disposed between higher and lower doped regions ofthe second region of the optical waveguide to reduce optical loss of theoptical beam.
 2. The method of claim 1 wherein modulating theconcentration of free charge carriers in the charge modulated regioncomprises modulating the concentration of free charge carriers proximateto the insulating region disposed between the first and second regionsof the optical waveguide.
 3. The method of claim 1 further comprisingimproving an electrical coupling between a first contact and the opticalwaveguide with a first higher doped portion of the second region of theoptical waveguide coupled to the first contact.
 4. The method of claim 3further comprising isolating the first contact from the optical paththrough which the optical beam is directed with a first buffer ofinsulating material disposed along the optical waveguide between thefirst contact and the optical path of the optical beam.
 5. The method ofclaim 4 further comprising improving an electrical coupling between asecond contact and the optical waveguide with a second higher dopedportion of the second region of the optical waveguide coupled to thesecond contact.
 6. The method of claim 5 further comprising isolatingthe second contact from the optical path through which the optical beamis directed with a second buffer of insulating material disposed alongthe optical waveguide between the second contact and the optical path ofthe optical beam.